Add assertions to check that media sources are not prepared twice.

This lets apps fail-fast when they try to reuse media source instances.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=180934445

library/core/src/main/java/com/google/android/exoplayer2/source/ClippingMediaSource.java

@@ -131,7 +131,8 @@ public final class ClippingMediaSource implements MediaSource, MediaSource.Liste
 
   @Override
   public void prepareSource(ExoPlayer player, boolean isTopLevelSource, Listener listener) {
-    this.sourceListener = listener;
+    Assertions.checkState(sourceListener == null, MEDIA_SOURCE_REUSED_ERROR_MESSAGE);
+    sourceListener = listener;
     mediaSource.prepareSource(player, false, this);
   }
 

library/core/src/main/java/com/google/android/exoplayer2/source/ConcatenatingMediaSource.java

@@ -89,6 +89,7 @@ public final class ConcatenatingMediaSource implements MediaSource {
 
   @Override
   public void prepareSource(ExoPlayer player, boolean isTopLevelSource, Listener listener) {
+    Assertions.checkState(this.listener == null, MEDIA_SOURCE_REUSED_ERROR_MESSAGE);
     this.listener = listener;
     if (mediaSources.length == 0) {
       listener.onSourceInfoRefreshed(this, Timeline.EMPTY, null);

library/core/src/main/java/com/google/android/exoplayer2/source/DynamicConcatenatingMediaSource.java

@@ -331,6 +331,7 @@ public final class DynamicConcatenatingMediaSource implements MediaSource, Playe
   @Override
   public synchronized void prepareSource(ExoPlayer player, boolean isTopLevelSource,
       Listener listener) {
+    Assertions.checkState(this.listener == null, MEDIA_SOURCE_REUSED_ERROR_MESSAGE);
     this.player = player;
     this.listener = listener;
     preventListenerNotification = true;

library/core/src/main/java/com/google/android/exoplayer2/source/ExtractorMediaSource.java

@@ -325,6 +325,7 @@ public final class ExtractorMediaSource implements MediaSource, ExtractorMediaPe
 
   @Override
   public void prepareSource(ExoPlayer player, boolean isTopLevelSource, Listener listener) {
+    Assertions.checkState(sourceListener == null, MEDIA_SOURCE_REUSED_ERROR_MESSAGE);
     sourceListener = listener;
     notifySourceInfoRefreshed(C.TIME_UNSET, false);
   }
@@ -356,7 +357,7 @@ public final class ExtractorMediaSource implements MediaSource, ExtractorMediaPe
 
   @Override
   public void releaseSource() {
-    sourceListener = null;
+    // Do nothing.
   }
 
   // ExtractorMediaPeriod.Listener implementation.

library/core/src/main/java/com/google/android/exoplayer2/source/LoopingMediaSource.java

@@ -36,6 +36,7 @@ public final class LoopingMediaSource implements MediaSource {
   private final int loopCount;
 
   private int childPeriodCount;
+  private boolean wasPrepareSourceCalled;
 
   /**
    * Loops the provided source indefinitely. Note that it is usually better to use
@@ -61,6 +62,8 @@ public final class LoopingMediaSource implements MediaSource {
 
   @Override
   public void prepareSource(ExoPlayer player, boolean isTopLevelSource, final Listener listener) {
+    Assertions.checkState(!wasPrepareSourceCalled, MEDIA_SOURCE_REUSED_ERROR_MESSAGE);
+    wasPrepareSourceCalled = true;
     childSource.prepareSource(player, false, new Listener() {
       @Override
       public void onSourceInfoRefreshed(MediaSource source, Timeline timeline, Object manifest) {

library/core/src/main/java/com/google/android/exoplayer2/source/MediaSource.java

@@ -149,6 +149,8 @@ public interface MediaSource {
 
   }
 
+  String MEDIA_SOURCE_REUSED_ERROR_MESSAGE = "MediaSource instances are not allowed to be reused.";
+
   /**
    * Starts preparation of the source.
    * <p>

library/core/src/main/java/com/google/android/exoplayer2/source/MergingMediaSource.java

@@ -19,6 +19,7 @@ import android.support.annotation.IntDef;
 import com.google.android.exoplayer2.ExoPlayer;
 import com.google.android.exoplayer2.Timeline;
 import com.google.android.exoplayer2.upstream.Allocator;
+import com.google.android.exoplayer2.util.Assertions;
 import java.io.IOException;
 import java.lang.annotation.Retention;
 import java.lang.annotation.RetentionPolicy;
@@ -97,6 +98,7 @@ public final class MergingMediaSource implements MediaSource {
 
   @Override
   public void prepareSource(ExoPlayer player, boolean isTopLevelSource, Listener listener) {
+    Assertions.checkState(this.listener == null, MEDIA_SOURCE_REUSED_ERROR_MESSAGE);
     this.listener = listener;
     for (int i = 0; i < mediaSources.length; i++) {
       final int sourceIndex = i;

library/core/src/main/java/com/google/android/exoplayer2/source/SingleSampleMediaSource.java

@@ -158,6 +158,8 @@ public final class SingleSampleMediaSource implements MediaSource {
   private final boolean treatLoadErrorsAsEndOfStream;
   private final Timeline timeline;
 
+  private boolean isPrepared;
+
   /**
    * @param uri The {@link Uri} of the media stream.
    * @param dataSourceFactory The factory from which the {@link DataSource} to read the media will
@@ -251,6 +253,8 @@ public final class SingleSampleMediaSource implements MediaSource {
 
   @Override
   public void prepareSource(ExoPlayer player, boolean isTopLevelSource, Listener listener) {
+    Assertions.checkState(!isPrepared, MEDIA_SOURCE_REUSED_ERROR_MESSAGE);
+    isPrepared = true;
     listener.onSourceInfoRefreshed(this, timeline, null);
   }
 

library/core/src/main/java/com/google/android/exoplayer2/source/ads/AdsMediaSource.java

@@ -204,6 +204,7 @@ public final class AdsMediaSource implements MediaSource {
   @Override
   public void prepareSource(final ExoPlayer player, boolean isTopLevelSource, Listener listener) {
     Assertions.checkArgument(isTopLevelSource);
+    Assertions.checkState(this.listener == null, MEDIA_SOURCE_REUSED_ERROR_MESSAGE);
     this.listener = listener;
     this.player = player;
     playerHandler = new Handler();

library/dash/src/main/java/com/google/android/exoplayer2/source/dash/DashMediaSource.java

@@ -483,6 +483,7 @@ public final class DashMediaSource implements MediaSource {
 
   @Override
   public void prepareSource(ExoPlayer player, boolean isTopLevelSource, Listener listener) {
+    Assertions.checkState(sourceListener == null, MEDIA_SOURCE_REUSED_ERROR_MESSAGE);
     sourceListener = listener;
     if (sideloadedManifest) {
       loaderErrorThrower = new LoaderErrorThrower.Dummy();

library/hls/src/main/java/com/google/android/exoplayer2/source/hls/HlsMediaSource.java

@@ -324,7 +324,7 @@ public final class HlsMediaSource implements MediaSource,
 
   @Override
   public void prepareSource(ExoPlayer player, boolean isTopLevelSource, Listener listener) {
-    Assertions.checkState(playlistTracker == null);
+    Assertions.checkState(sourceListener == null, MEDIA_SOURCE_REUSED_ERROR_MESSAGE);
     playlistTracker = new HlsPlaylistTracker(manifestUri, dataSourceFactory, eventDispatcher,
         minLoadableRetryCount, this, playlistParser);
     sourceListener = listener;
@@ -361,7 +361,6 @@ public final class HlsMediaSource implements MediaSource,
       playlistTracker.release();
       playlistTracker = null;
     }
-    sourceListener = null;
   }
 
   @Override

library/smoothstreaming/src/main/java/com/google/android/exoplayer2/source/smoothstreaming/SsMediaSource.java

@@ -419,6 +419,7 @@ public final class SsMediaSource implements MediaSource,
 
   @Override
   public void prepareSource(ExoPlayer player, boolean isTopLevelSource, Listener listener) {
+    Assertions.checkState(sourceListener == null, MEDIA_SOURCE_REUSED_ERROR_MESSAGE);
     sourceListener = listener;
     if (manifest != null) {
       manifestLoaderErrorThrower = new LoaderErrorThrower.Dummy();
@@ -455,7 +456,6 @@ public final class SsMediaSource implements MediaSource,
 
   @Override
   public void releaseSource() {
-    sourceListener = null;
     manifest = null;
     manifestDataSource = null;
     manifestLoadStartTimestamp = 0;